The invention relates to an integral vitreous article composed of an opaque portion encased within a thin surface layer of transparent glass containing a titania-coupled colorant. The opaque portion contains light scattering particles or crystals and may be in the nature of either an opacified glass or a spontaneously-formed glass-ceramic. A specific embodiment of the invention is a pressed dinner plate having a transparent edge. The invention further relates to a method of making an article by simultaneously molding and surface quenching a mass of suitable molten glass.
A glass may be opacified by dispersing therein particles of such a nature that visible light is scattered or diffused, rather than directly transmitted. In general, the ability of a particle to scatter light, and thus impart opacity, depends on the degree of difference between the refractive indices of the particle and the glass, particle size, and particle concentration. Commonly, opacifying particles may be a few thousand Angstrom units in diameter and constitute no more than about 10% of the glass volume.
Spontaneously-formed glass-ceramics are characterized by the separation of a crystal phase from a glass as it cools, that is, without the further heat treatment normally required for the separation of such crystal phase. Further information on spontaneous glass-ceramics and their formation is found in U.S. Pat. No. 3,985,532 (Grossman), No. 3,985,533 (Grossman), and No. 4,000,998 (Rittler).
The role of glass colorants, such as the oxides of nickel, iron, cobalt, chromium and maganese, and the colored glasses thus produced, has been the subject of much patent and other technical literature. A comprehensive reference on the subject is the monograph "Coloured Glasses" by W. A. Weyl, reprinted in 1959 by Dawson's of Pall Mall (London). It has been recognized that the known glass colorants may produce a different color effect in glass-ceramics, that is, nucleated and crystallized glasses. Typical United States patents describing color effects in glass-ceramic materials are U.S. Pat. No. 3,788,865 (Babcock et al.) and my earlier U.S. Pat. No. 4,009,042.